Tiny: Support if Value is greatest, Interceptor if Threat is greatest, Guardian if Fortitude is greatest

Small: Support if Value is greatest, Assault if Threat is greatest, Escort if Fortitude is greatest

Medium: Support if Value is greatest, Assault if Threat is greatest, Escort if Fortitude is greatest

Large: Support if Value is greatest, Capital if Threat is greatest, Escort if Fortitude is greatest

Huge: Support if Value is greatest, otherwise Capital

And, of course, you can override the role designation in the ship designer.

#all modules have a fortitude stat, a value stat and a threat stat (though in many cases it is zero). These are summed for the whole ship and then it's role is determined using roughly the table that Petri outlines above.